Xylitol bioproduction in hemicellulosic hydrolysate obtained from sorghum forage biomass

Integrated bioinformatics, modelling, and gene expression analysis of the putative pentose transporter from Candida tropicalis during xylose fermentation with and without glucose addition

The transport of substrates across the cell membrane plays an essential role in nutrient assimilation by yeasts. The establishment of an efficient microbial cell factory, based on the maximum use of available carbon sources, can generate new technologies that allow the full use of lignocellulosic constituents. These technologies are of interest because they could promote the formation of added-value products with economic feasibility. In silico analyses were performed to investigate gene sequences capable of encoding xylose transporter proteins in the Candida tropicalis genome. The current study identified 11 putative transport proteins that have not yet been functionally characterized. A phylogenetic tree highlighted the potential C. tropicalis xylose-transporter proteins CtXUT1, CtXUT4, CtSTL1, CtSTL2, and CtGXT2, which were homologous to previously characterized and reported xylose transporters. Their expression was quantified through real-time qPCR at defined times, determined through a kinetic analysis of the microbial growth curve in the absence/presence of glucose supplemented with xylose as the main carbon source. The results indicated different mRNA expression levels for each gene. CtXUT1 mRNA expression was only found in the absence of glucose in the medium. Maximum CtXUT1 expression was observed in intervals of the highest xylose consumption (21 to 36 h) that corresponded to consumption rates of 1.02 and 0.82 g/L/h in the formulated media, with xylose as the only carbon source and with glucose addition. These observations indicate that CtXUT1 is an important xylose transporter in C. tropicalis. KEY POINTS: • Putative xylose transporter proteins were identified in Candida tropicalis; • The glucose concentration in the cultivation medium plays a key role in xylose transporter regulation; • The transporter gene CtXUT1 has an important role in xylose consumption by Candida tropicalis.

Xylitol Production from Exhausted Olive Pomace by Candida boidinii

In this work, the production of xylitol from a hemicellulosic hydrolysate of exhausted olive pomace (EOP), a residue originated in the olive oil production process by Candida boidinii, was assessed. The hydrolysate was obtained by dilute acid pretreatment of EOP at 170 °C and 2% H2SO4 (w/v). A previous detoxification step of the hydrolysate was necessary, and its treatment with activated charcoal and ion-exchange resin was evaluated. Prior to fermentation of the hydrolysate, fermentation tests in synthetic media were performed to determine the maximum xylitol yield and productivity that could be obtained if inhibitory compounds were not present in the medium. In addition, the glucose existing in the media exerted a negative influence on xylitol production. A maximum xylitol yield of 0.52 g/g could be achieved in absence of inhibitor compounds. Fermentation of the hemicellulosic hydrolysate from EOP after detoxification with ion-exchange resin resulted in a xylitol yield of 0.43 g/g.

Hydrolysis of Corncob Hemicellulose by Solid Acid Sulfated Zirconia and Its Evaluation in Xylitol Production

Corncob is an abundant agricultural residue containing high content of hemicellulose. In this paper, the hemicellulosic hydrolysate was prepared from the hydrolysis of corncob using the solid acid sulfated zirconia as a catalyst. According to response surface analysis experiments, the optimum conditions for preparing hemicellulosic hydrolysate catalyzed by sulfated zirconia were determined as follows: solid (sulfated zirconia)-solid (corncob) ratio was 0.33, solid (corncob)-liquid (water) ratio was 0.09, temperature was 153 °C, and time was 5.3 h. Under the optimized conditions, the soluble sugar concentration was 30.12 g/L with a yield of 033 g/g corncob. Subsequently, xylitol production from the resulting hemicellulosic hydrolysate was demonstrated by Candida tropicalis, and results showed that the yield of xylitol from the hemicellulosic hydrolysate could be significantly improved on a basis of decolorization and detoxification before fermentation. The maximum yield of xylitol from the hemicellulosic hydrolysate fermented by C. tropicalis was 0.76 g/g. This study provides a new attempt for xylitol production from the hemicellulosic hydrolysate.

Valorization of apple pomace using bio-based technology for the production of xylitol and 2G ethanol

Apple pomace was studied as a raw material for the production of xylitol and 2G ethanol, since this agroindustrial residue has a high concentration of carbohydrate macromolecules, but is still poorly studied for the production of fermentation bioproducts, such as polyols. The dry biomass was subjected to dilute-acid hydrolysis with H₂SO₄ to obtain the hemicellulosic hydrolysate, which was concentrated, detoxified and fermented. The hydrolyzate after characterization was submitted to submerged fermentations, which were carried out in Erlenmeyer flasks using, separately, the yeasts Candida guilliermondii and Kluyveromyces marxianus. High cellulose (32.62%) and hemicellulose (23.60%) contents were found in this biomass, and the chemical hydrolysis yielded appreciable quantities of fermentable sugars, especially xylose. Both yeasts were able to metabolize xylose, but Candida guilliermondii produced only xylitol (9.35 g L^-1 in 96 h), while K. marxianus produced ethanol as the main product (10.47 g L^-1 in 24 h) and xylitol as byproduct (9.10 g L^-1 xylitol in 96 h). Maximum activities of xylose reductase and xylitol dehydrogenase were verified after 24 h of fermentation with C. guilliermondii (0.23 and 0.53 U/mg_prot, respectively) and with K. marxianus (0.08 e 0.08 U/mg_prot, respectively). Apple pomace has shown potential as a raw material for the fermentation process, and the development of a biotechnological platform for the integrated use of both the hemicellulosic and cellulosic fraction could add value to this residue and the apple production chain.

Xylitol Separation from a Polyol Mixture Using Lanthanide Ion-loaded Resins

Xylitol separation from a polyol mixture of the byproducts from bioethanol production processes was performed by liquid chromatography using short columns packed with lanthanide ion-loaded ion-exchange resins. Xylitol was successfully separated with sufficiently high resolution using adsorbents with medium rare-earth metal ions, such as Nd³⁺ and Sm³⁺. The adsorbents' specific nature is explained by the so-called "gadolinium break," which is known as a discontinuous behavior of thermodynamic parameters in complexation of the lanthanide series. From the observed behavior, the optimum lanthanide ions could be chosen to prepare appropriate adsorbents for ligand-exchange chromatography of given polyol mixtures.

Xylose-Enriched Ethanol Fermentation Stillage from Sweet Sorghum for Xylitol and Astaxanthin Production

Developing integrated biorefineries requires the generation of high-value co-products produced alongside cellulosic ethanol. Most industrial yeast strains produce ethanol at high titers, but the small profit margins for generating ethanol require that additional high-value chemicals be generated to improve revenue. The aim of this research was to boost xylose utilization and conversion to high-value co-products that can be generated in an integrated biorefinery. Pretreated sweet sorghum bagasse (SSB) was hydrolyzed in sweet sorghum juice (SSJ) followed by ethanol fermentation. Ethanol was removed from the fermentation broth by evaporation to generate a stillage media enriched in xylose. Candida mogii NRRL Y-17032 could easily grow in non-detoxified stillage media, but a high xylitol yield of 0.55 g xylitol/g xylose consumed was achieved after recovered cells were resuspended in synthetic media containing supplemented xylose. Phaffia rhodozyma ATCC 74219 could be cultivated in non-detoxified stillage media, but astaxanthin generation was increased 4-fold (from 17.5 to 71.7 mg/L) in detoxified media. Future processing strategies to boost product output should focus on a two-step process where the stillage media is used as the growth stage, and a synthetic media for the production stage utilizing xylose generated from SSB through selective hemicellulase enzymes.

Xylitol production by Debaryomyces hansenii and Candida guilliermondii from rapeseed straw hemicellulosic hydrolysate

This study evaluated the possibility of using rapeseed straw hemicellulosic hydrolysate as a fermentation medium for xylitol production. Two yeast strains, namely Debaryomyces hansenii and Candida guilliermondii, were used for this bioconversion process and their performance to convert xylose into xylitol was compared. Additionally, different strategies were evaluated for the hydrolysate detoxification before its use as a fermentation medium. Assays in semi-defined media were also performed to verify the influence of hexose sugars on xylose metabolism by the yeasts. C. guilliermondii exhibited higher tolerance to toxic compounds than D. hansenii. Not only the toxic compounds present in the hydrolysate affected the yeast's performance, but glucose also had a negative impact on their performance. It was not necessary to completely eliminate the toxic compounds to obtain an efficient conversion of xylose into xylitol, mainly by C. guilliermondii (Y_P/S=0.55g/g and 0.45g/g for C. guilliermondii and D. hansenii, respectively).